1. Field of the Invention
This invention relates to an in-motion, slit radiographic system and a method for non-destructive inspection of materials.
2. Description of the Prior Art
The extreme environment in which space vehicles operate creates tremendous demands on the materials used in the construction of those vehicles. As a result of such demands in recent years there has been extensive development and fabrication of advanced composite materials for re-entry vehicles and rocket nozzles. These composite materials are especially attractive because of improved strength to weight ratio, resistance to crack propagation, uniformity of ablation and ability to tailor the directional properties of the composite materials. In one form of such composite materials a 3-dimensional fibrous composite is produced by weaving or winding techniques followed by high pressure impregnation steps, and as a result, the weaved fiber parameters, such as spacing waviness and distortion, are very important to monitor and maintain. Due to the high cost of machining space vehicle parts from fibrous composite materials, it is very desirable to use non-destructive testing of the composite material to accurately determine the weave parameters and detect defects before machining of the part actually takes place.
One of the most useful of the non-destructive testing techniques is the use of radiography to obtain an X-ray image of the composite structure on radiographic film. Unfortunately, 3-dimensional composite materials present a new set of problems in the radiographic evaluation of structures, inasmuch as the chief concerns are weave geometry, spacing of fiber bundles, waviness, distortion and uniformity of density. As a result, 3-dimensional radiographic technique development has been concentrated on obtaining very defined X-ray images for evaluating the weave and impregnation.
Due to the highly oriented structure and fairly large cross-sections of these 3-dimensional composite materials, standard radiographic techniques display significant distortions due to the non-parallel rays of an X-ray beam emitted from an X-ray source. As a result, it is difficult to interpret the radiographs, because the non-parallel rays cause fuzziness and shadows on the radiograph.
One of the most popular and successful techniques currently used to eliminate the problems of non-parallel rays of X-ray radiation is what is known as slit radiography. Slit radiography refers to the use of a slit or opening in a shield to only allow a narrow beam of the radiation emitted from an X-ray source to impinge upon the object being tested. The rays within this narrow beam do not significantly diverge and are more nearly parallel to each other in comparison to the broad span of radiation emitted from an X-ray source. The shield prevents the widely diverging rays outside this narrow beam from impinging upon the object being tested. The use of slit radiography results in a relatively undistorted image of the object being tested, and is well known to those skilled in the art.
While the size of the slit used in slit radiography can be varied, generally, the use of a larger slit results in more diverging or non-parallel rays impinging on the object being tested and, therefore, a more distorted radiograph. Conversely, the use of a smaller slit size results in lessening the distortion of the radiograph. However, if a small slit size is used, many individual radiographs must be made in order to X-ray the entire object being tested. Development of X-ray film is time-consuming and expensive, and it is far more desirable to expose and develop a small number of radiographs as opposed to a large number.
The problem of developing multiple radiographs is resolved in the industry by what is known as in-motion slit radiography. Generally, with in-motion slit radiography, the X-ray film is attached to the object being tested, and the object itself is moved underneath the slit. The X-rays passing through this slit eventually strike the entire surface of the object when the motion is completed. A similar technique is to keep the object and the film stationary and move the X-ray source and the slit over the surface of the object being tested.
When in-motion radiography combined with slit radiography provides a reasonable useful and efficient method for X-raying composite structures, the large numbers of sizes and shapes of objects which must be tested create a need to devise numerous slit sizes, slit shapes and various types of motion in order to obtain satisfactory radiographs.